G6b-B inhibits constitutive and agonist-induced signaling by glycoprotein VI and CLEC-2.
Platelets play an essential role in wound healing by forming thrombi that plug holes in the walls of damaged blood vessels. To achieve this, platelets express a diverse array of cell surface receptors and signaling proteins that induce rapid platelet activation. In this study we show that two platelet glycoprotein receptors that signal via an immunoreceptor tyrosine-based activation motif (ITAM) or an ITAM-like domain, namely the collagen receptor complex glycoprotein VI (GPVI)-FcR gamma-chain and the C-type lectin-like receptor 2 (CLEC-2), respectively, support constitutive (i.e. agonist-independent) signaling in a cell line model using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay that can detect low level activation of phospholipase Cgamma (PLCgamma). Constitutive and agonist signaling by both receptors is dependent on Src and Syk family kinases, and is inhibited by G6b-B, a platelet immunoglobulin receptor that has two immunoreceptor tyrosine-based inhibitory motifs in its cytosolic tail. Mutation of the conserved tyrosines in the two immunoreceptor tyrosine-based inhibitory motifs prevents the inhibitory action of G6b-B. Interestingly, the inhibitory activity of G6b-B is independent of the Src homology 2 (SH2)-domain containing tyrosine phosphatases, SHP1 and SHP2, and the inositol 5'-phosphatase, SHIP. Constitutive signaling via Src and Syk tyrosine kinases is observed in platelets and is associated with tyrosine phosphorylation of GPVI-FcR gamma-chain and CLEC-2. We speculate that inhibition of constitutive signaling through Src and Syk tyrosine kinases by G6b-B may help to prevent unwanted platelet activation.
Mori
Jun
J
Centre for Cardiovascular Sciences, Institute of Biomedical Research, University of Birmingham, B15 2TT, UK.
Pearce
Andrew C
AC
Spalton
Jennifer C
JC
Grygielska
Beata
B
Eble
Johannes A
JA
Tomlinson
Michael G
MG
Senis
Yotis A
YA
Watson
Steve P
SP
D020816:Amino Acid Motifs / Q000502:physiology; D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D002460:Cell Line; D002645:Chickens; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D037181:Lectins, C-Type / Q000235:genetics / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D050778:NFATC Transcription Factors / Q000235:genetics / Q000378:metabolism; D051966:Phospholipase C gamma / Q000235:genetics / Q000378:metabolism; D015539:Platelet Activation / Q000502:physiology*; D010980:Platelet Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D017452:Receptors, IgG / Q000235:genetics / Q000378:metabolism; D011971:Receptors, Immunologic / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D000072377:Syk Kinase; D019061:src-Family Kinases / Q000235:genetics / Q000378:metabolism
C104778:CLEC2B protein, human; D047908:Intracellular Signaling Peptides and Proteins; D037181:Lectins, C-Type; C119883:MPIG6B protein, human; D008562:Membrane Glycoproteins; D050778:NFATC Transcription Factors; D010980:Platelet Membrane Glycoproteins; D020033:Protein Isoforms; D017452:Receptors, IgG; D011971:Receptors, Immunologic; C098341:platelet membrane glycoprotein VI; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; D019061:src-Family Kinases; D051966:Phospholipase C gamma